Slice open the tenderloin lengthways, two-thirds of the way through. Open out the split tenderloin and pound it gently with a rolling pin, to flatten and widen it slightly.
For the stuffing, melt the butter and fry the shallots and sage for about 10 minutes. Add the spinach and cook until it wilts, then mix in the rest of the stuffing ingredients.
Spread the stuffing along the centre of the tenderloin, patting it down firmly. Roll up the tenderloin and tie it with kitchen string at roughly 4cm intervals.
Lay the meat in a roasting tin, cover with the slices of bacon and surround with the onions and rosemary. Drizzle the olive oil over the meat and onions.
Roast for 35-40 minutes, basting it a few times during cooking. Once cooked, remove the tenderloin, cover with foil and rest for about 5 minutes.
Meanwhile, discard the onions and the rosemary and deglaze the pan with wine. Add the stock and boil rapidly until reduced by half. Strain.
Cut the string off the tenderloin and then slice it slightly on the diagonal. Serve immediately with the hot gravy.